1. Field of the Invention
The present invention relates to a control circuit for generating a tone and in particular to a control circuit that is adapted to generate a chime sound preferably with an exponential decay.
2. Discussion
Tone generators were previously incorporated on automobiles as a result of the Imposition of Federal regulations requiring four to eight second audible warning signals whenever an automobile is operated without the seat belts properly fastened. Initially, these devices consisted of a buzzer unit controlled by a bi-metallic timer circuit. Before long, the same buzzer unit was further utilized to provide audible signal indications of other monitored vehicle conditions. For example, a single buzzer would sound when a user left a key in a vehicle ignition, or when headlamps were left on after a vehicle ignition was turned off.
However, it soon became apparent that several problems existed with the aforementioned buzzer units, namely, the constant buzzer sound was annoying, and production of the same audible tone for each of a plurality of conditions proved confusing to an operator. Therefore, a variety of devices were developed utilizing readily distinguishable audible signals for each of the conditions monitored in a vehicle. Likewise, tone generators were tailored to produce varying types and frequencies of signals, for example, a chime signal, a pulse tone, or a steady tone, depending on the particular enabling signal which was monitored. After a period of use by customers, it became apparent that the chime sound was the least annoying and most pleasing to the customer, and various implementations were developed.
A variety of devices were subsequently developed which are presently in use for driving tone generators which produce sounds that are pleasing to the human ear. Typically, a control circuit is used which drives a speaker to generate a tone. Preferably, an electro-mechanical or a piezo-electric speaker is utilized with the control circuit. To reproduce a single chime strike, the control circuit must first produce the tone of the desired chime, then gradually decrease the tone's amplitude from an initial loud value to zero. The control circuit must also restart this process periodically if a repetitive chime sound is desired. Various techniques have been utilized to produce the chime tone, control the tone amplitude, and fashion a repetitive chime from individual chime strikes. For example, pulse width modulation has been used to control the tone's amplitude. Similarly, tones have been generated from reconditioned square wave signals provided by simple logic gate oscillator circuits.
Another technique for synthesizing an amplitude modulated tone signal employs resistor-capacitor circuits, or RC networks. The capacitor is rapidly charged and subsequently slowly discharges through the resistor to produce a voltage waveform. This waveform amplitude modulates the tone signal to provide a high quality chime. A desired waveform can be fashioned by properly selecting values for the capacitor and resistor, and by appropriately controlling the voltage applied to the resulting network. The discharge rate of the capacitor is influenced by the impedance of components to which the output of the RC network is applied, for example an amplifier and speaker. Unless the impedance of the output components is kept high, leakage from the capacitor through these components will alter the waveform produced by the RC network resulting in a highly distorted sound. Although this circuit can produce a desired chime, the ability to produce a multitude of different chimes having different frequencies and different decay rates is somewhat limited and requires formation of alternative circuit elements which are selectively electrically coupled to produce a desired particular chime.